The present invention relates to system for measuring delay time variation in optical fiber digital communication system to prevent loss or slipping of information in a transmission system using optical fiber cable including an optical regenerative repeater.
Optical communication systems have drawn the attention of the public in recent years since it can handle enormous amounts of communication data. Because they are light in weight and economical, optical fiber cables are used as a transmission media in the optical communication system. However, optical fiber cables tend to easily expand, contract and bend depending upon external conditions. Especially expansion and contraction of optical fiber cables due to changes in external temperature will cause slippage or loss of information signal at multiplex end office during time-division multiplex communication.
When information signals which have been transmitted from two or more different points are going to be time-division multiplexed, expansion or contraction of the optical fiber cable from each point changes the propagation delay. When the phase difference of the information signal to the multiplex clock fluctuates more than 2 radian, such information slippage or loss occurs.
Therefore, various systems to detect variation in propagation delay time have been studied to compensate for this variation in propagation delay time.
As optical fiber submarine cable communication requires more reliability and economic efficiency than land optical fiber communication system, the propagation delay time variation detecting system is described taking the former as an example.
FIG. 5 shows one of the conventional transmission systems which economically provide communication among three or more signal transmitters and receivers (hereinafter called "Node"). For example, node A sends time-shared multiplexed information to node B and node C by the transmitter (1) via a fiber (3) and repeaters R.sub.1, ---, and R.sub.L. The signal branch unit (4) multiplexes and/or de-multiplexes signals and returns the signal via fiber (3'), repeaters R'.sub.L ---, and R'.sub.1 to node A, and the receiver (2) releases the multiplication. (Sakaguchi et al, Journal of Electronics and Communication in Japan CS84-123, on pages 1 to 8, 1984)
In this example, since the signal branch unit (4) is installed in the sea, the clock source is not contained in said device (4) from a standpoint of equipment complexity, and therefore a communication reference clock signal is used as a clock signal for multiplexing, de-multiplexing in said branch unit (4). Therefore, the phase difference between a clock signal and a communication signal changes with time in said branch unit (4). Whenever the phase difference exceeds 2.pi. radians, information loss or slippage occurs. The slippage causes the frame to be out of synchronization at a receive node. It is impossible to transmit correct information between each node until the frame synchronization is established.
The slippage occurs even in land cable when the delay time changes depending upon temperature change, pressure, change of wavelength of a light source. To avoid this slippage, a delay variation compensation is required. Three prior systems for the compensation are described below.
The first prior system is a pulse stuffing system which uses a clock signal with frequency higher than a communication clock signal. The system has a buffer memory to cancel delay time variation and adjust the number of insertion pulses to establish the synchronization of the two systems.
The second prior system has an independent clock source in branch unit to which each nodes are synchronized. The branch unit detects the phase difference between the reference clock signal in the branch unit itself and the receive clock signal to provide the delay time variation. The delay time compensation is carried out by using the measured phase difference, and thus the slippage or loss of information is prevented.
The third prior system uses the communication clock signal for multiplexing clock signal. For example, in a branch unit (4) in FIG. 5, clock signal which is extracted from communication signals from nodes C, A and B respectively is used as multiplexing clock signal for sending signals to those nodes. This system enables each node to compare the phase between transmission clock signal and receive clock signal, whereby nodes A, B and C can detect delay time variation between each nodes. The measured phase differences are informed to other nodes to calculate delay time variation between each node and device (4) so that the delay time compensation is performed.
The conventional examples to avoid loss or slippage have been described. We describe the problems when they are applied to an optical fiber submarine cable network system equipped with a time-division signal branch unit which is installed in the sea.
In conventional first and second examples, a stable and highly reliable clock source is required for multiplexing at the branch unit.
Though there have been currently atomic oscillators using cesium (Cz) and rubidium (Rb) as stable clock source, double or triple redundant structure is required to obtain high operational reliability when these oscillators are used. Accordingly the scale of equipment will be larger and it is not adequate to apply them to signal branch unit (4) in the submarine branch optical network system. In addition, the control for stuffing will be larger in the first examples.
In second and third examples, a variable delay circuit is required. For that purpose, a variable length coaxial pipe or other similar goods which expand mechanically must be used to adjust delay because it is difficult at present to use other variable delay circuits for high speed signals.
A coaxial pipe and variable delay device including a drive unit will be so large that it is difficult to mount them in the submarine signal branch unit (4). Instead they must be installed in end terminals for each node installed on land. In the second example, it is possible to compensate for delay at each node on land. However, to do this, a means for informing each node the delay time variation in a submarine signal branch unit (4) will be required, resulting in large signal branch unit (4).
In third example, it is possible to adjust delay at the node concerned without increasing the size of the submarine signal branch unit (4). However, this system has the following disadvantages: (i) a complicated operation of sending and receiving delay variation information between each node is required. (ii) Whe there is something wrong in a node, the measure of the delay time variation becomes impossible in the whole cable system.